Xylan is a major component of hemicellulose found predominantly in plant cell walls. Endo-xylanases (E.C. 3.2.1.8) are able to randomly hydrolyze the beta (1-4) glycosidic bonds between xylose residues making up the backbone of xylans. The xylanase enable plant structural polysaccharide to be hydrolyzed, and these products can be exploited as a rich source of carbon and energy for the growth of herbivores and microorganisms.
The plant cell wall consists largely of polysaccharides and contains lesser amounts of lignin and protein. The major polysaccharide components of plant cell walls are cellulose, hemicellulose, and pectin (Saha 2003). Fibrils of cellulose embedded in a matrix of pectin, hemicellulose (comprising various .beta.-xylan polymers), phenolic esters and protein produce a protective structure resistant to dehydration and penetration by phytopathogens through mechanical and enzymatic mechanisms. It represents a rich source of an important renewable resource utilized by the pulp and paper, lumber, food, and biofuel industries (Beg, Kapoor et al. 2001; Lachke 2002; Saha 2003; Bajpai 2004).
The plant structural polysaccharides provide an important protection for plant and useful applications for human, but these components also hinder men from much utilization of plant products. For example, cereals are a major component of diets fed to mono-gastric animals, the endosperm cell wall of cereals containing non-starch polysaccharide (NSP)(Engberg, Hedemann et al. 2004). The animals do not synthesize the enzymes capable of degrading these structural polysaccharides (e.g. hemicellulose), and as a result, these undigested NSP can often be problematic for mono-gastric animals being fed such a diet, causing intestinal disturbances, typified by sticky droppings and poor growth in young animals. It has been demonstrated previously that the anti-nutritive effects of NSP are related to their propensity to form high molecular-weight viscous aggregates in the gastrointestinal tract (Choct and Annison 1992). The problems and bad effects of hemicellulose also can be found in pulp making, pulp and juice production (Beg, Kapoor et al. 2001).
Hemicellulose, the second most prevalent polysaccharide in many plant cell walls is composed mainly of xyloglucan or xylan polymers. Xylans have a backbone structure of .beta.(1-4)-linked xylose residues. The structure of xylan is complicated by the attachment of various side chains (e.g., acetic acid, arabinose, coumaric acid, ferulic acid, glucuronic acid, 4-O-methylglucuronic acid) to the xylose residues (Saha 2003). The strands of hemicellulose are hydrogen bonded to cellulose fibrils to form a strong interconnected lattice. Cell wall composition varies with plant species, tissue type, growth conditions, and age.
Degradation of the plant cell wall is complicated by the structure of polysaccharides. Cellulose is a linear glucose polymer of β(1-4)-linkage and requires the synergistic hydrolysis of endoglucanase, and cellobiohydrolase and beta-glucosidase for complete degradation. In comparison, xylan is the most common in hemicellulosic polysaccharides. Xylan is a major polysaccharide comprising a backbone of xylose residues linked by β-1,4-glycosidic bonds. The main chain of xylan is composed of β-xylopyranose residues but highly substituted in its side chain, thus, xylan requires more and different enzymes, for complete degradation. An endoxylanase randomly cleaves the xylan backbone into xylooligosaccharides which are subsequently degraded to xylose by a xylosidase. Ferulic and p-coumaric acid crosslinks are degraded by feruloyl and p-coumaryl esterases. Substituents of xylan backbone are cleaved from the xylan backbone with arabinofuranidase, acetylxylan esterase and α-glucuronidase (Castanares 1992; Christov and Prior 1993; Saha 2003). Although various enzymes are necessary to the complete degradation, liquefaction of hemicellulose requires only the shortening of the xylan polymers. Consequently, this objective may be achieved by the production of xylooligosaccharides through the hydrolysis reaction of an endoxylanase (Beg, Kapoor et al. 2001).
Numerous applications of xylanases have been developed for many purposes. For instance, xylanases was used in biopulping to remove xylan impurities from cellulose pulps or to produce pulps with different characteristics. This green process is able to reduce the amount of chemical bleacher (chlorine) and the energy needed for refining pulp (Bajpai 2004). Xylanases can be the feeding enzyme, to improve the digestibility of cereal by poultry and swine fed on cereals with high arabinoxylan content (Beg, Kapoor et al. 2001; Bruyer, Giec et al. 2001; Cowieson, Hruby et al. 2005). Xylanases can be used in bioconversion involving the hydrolysis of xylan to xylooligosaccharides may not only serve as prebiotics for bifidobacteria (Howard, Gordon et al. 1995) but also provide an alternative and healthy sweetener for diabetics and portlies (Campbell, Fahey et al. 1997). Further, xylanases are useful in the retting of flax fibers, the clarification of fruit juices, the preparation of dextrans for use as food thickeners and the production of fluids and juices from plant materials (Beg, Kapoor et al. 2001).
Because of the important and potential applications of xylanases in industries, an important aspect of xylanase research is to obtain high activity and specification of xylanases. Consequently several bacteria and fungi have been selected for the sources of xylanase. Among xylanolytic microorganisms, rumen fungi are able to degrade the most-resistant plant cell-wall polymers, thus, the rumen fungal population represents a rich and underutilized source of novel enzymes with tremendous potential for industrial and agricultural applications. Those cellulases and xylanase produced by these fungi are among the most-active fibrolytic enzymes described to date, and many cellulase and xylanase genes have been cloned from specific strains such as Orpinomyces PC-2 (Li, Chen et al. 1997) and Neocallimastix frontalis SK (Huang, Huang et al. 2005). The recombinant products of the xylanase genes were presented highly active and specific activity of endoxylanase when expressed in E. coli. 
In view of the foregoing, there remains a need for low cost xylanases having biochemical characteristics well suited for use in biobleaching, baking, animal feeding supplements, and xylooligosaccharide production. These previous xylanase genes usually obtained from the specific strain from rumen by molecular biology based specific technologies such PCR amplification, cDNA library construction and screening. Thus, the isolation of microbes from rumen would become one of the limitations to future successes at attempting to isolate novel genes and to comprehend the fibrolytic systems from rumen ecosystem. Accordingly, it is of great importance to obtain genes encoding xylan-degrading enzymes from novel sources. To the best of our knowledge, however, it is estimated that more than 90% of the total microbial population can not be isolated by currently known methods. In order to overcome such a problem and avoid complicated microbe-isolated protocols, the present invention provides a method directly obtain mixed genomic DNA from unpurified ruminal microbes as a gene source without isolating the microorganisms.